Transcript Slide 1

Carbonyl Ligands - C≡O
Examples of neutral, binary metal carbonyls:
4
Ti
5
V(CO)6
6
Cr(CO)6
7
Mn2(CO)10
8
9
Fe(CO)5
Co2(CO)8
Fe2(CO)9
Co4(CO)12
10
11
Ni(CO)4
Cu
Fe3(CO)12
Nb
Mo(CO)6
Tc2(CO)10
Zr
Hf
Ta
W(CO)6
Re2(CO)10
Ru(CO)5
Rh4(CO)12
Ru3(CO)12
Rh6(CO)16
Pd
Ag
Os(CO)5
Ir4(CO)12
Pt
Au
Os3(CO)12
Molecular Orbital (MO) Diagram
Experimental Data Supporting Nature of MO’s in CO
Species
CO
CO*
Config
(5s)2
(5s)1
(5s)1(2p)1
C-O Å
1.13
1.11
S 1.24
T 1.21
nCO cm-1
2143
2184
1489
1715
Comment
5s MO is weakly antibonding
2p MO is strongly antibonding
Three types (two of which are important) of CO-Metal bonding
interactions:
M-C bond:
increases
increases
increases
C-O bond:
increases
decreases
decreases
nCO freq:
increases
decreases
decreases
Carbonyl Infrared (IR) Stretching Frequencies
• The position of the carbonyl bands in the IR depends mainly on the bonding
mode of the CO (terminal, bridging) and the amount of electron density on the
metal being p-backbonded to the CO.
• The number (and intensity) of the carbonyl bands observed depends on the
number of CO ligands present and the symmetry of the metal complex. There
are also secondary effects such as Fermi resonance and overtone interactions
that can complicate carbonyl IR spectra.
O C
O C
M
M
nCO IR (cm -1 )
O
O
C
C
M
M
M
M
free CO
terminal mode
2 bridging
3 bridging
2143
2120 - 1850
1850 - 1720
1730 - 1500
(for neutral metal complexes)
Electronic Effects on nCO
As the electron density on a
metal center increases, more pbackbonding to the CO ligand(s)
takes place. This further weakens
the C-O bond by pumping more
electron density into the formally
empty carbonyl p* orbital. This
increases the M-CO bond strength
making it more double-bond-like,
i.e., the resonance structure
M=C=O assumes more importance.
dx
d 10
d6
Complex
nCO cm-1
free CO
2143
[Ag(CO)]+
2204
Ni(CO)4
2060
[Co(CO)4]-
1890
[Fe(CO)4]2-
1790
[Mn(CO)6]+
2090
Cr(CO)6
2000
[V(CO)6]-
1860
Ph2 Ph 2
P
OC
OC
P
Fe
Fe
C
O
C
O
O
C
CO
CO
OC
C
O
2100
2000
1900
-1 )
Wave num be rs (cm
1800
2-
Ph 2
P
Fe
C
Fe
P
Ph 2
O
CO
C
O
O
C
OC
Ni 2( -CO)(CO) 2(dppm) 2
Ni
CO
Ni
P
-CO
+CO
Ni 2(CO) 4(dppm) 2
OC
OC
P
P
P
P
P
Ni
Ni
P
-CO
P
+CO
P
2 Ni(CO) 3( -dppm)
OC
1
OC
Ni
C
O
2000
1900
1800
Wavenumbers (cm -1)
1700
P
CO
CO
Ligand Electronic Effects on nCO
nCO cm-1
Complex
Mo(CO)3(PF3)3
2090, 2055
Mo(CO)3(PCl3)3
2040, 1991
Mo(CO)3[P(OMe)3]3
1977, 1888
Mo(CO)3(PPh3)3
1934, 1835
Mo(CO)3(NCCH3)3
1915, 1783
Mo(CO)3(triamine)3
1898, 1758
Mo(CO)3(pyridine)3
1888, 1746
Based on CO IR stretching frequencies, the following ligands can be ranked from
best p-acceptor to worst:
NO+ > CO > PF3 > RNC > PCl3 > P(OR)3 > PR3 > RCN > NH3
Semi-Bridging Carbonyls
~ 150
Unsymmetrical bridging form. p* system accepts
electron density from second metal center. Distortions
away from a linear M-CO (180°) or a symmetrically
bridging CO (120°). Typical M-CO angle around 150°
(but with considerable variations).
O
C
M
M
filled Fe d orbital
O
O
C
C
O
Fe
C
C
Cotton & Troup
JACS, 1974, 96, 1233
Fe
N
C
O
O
C
N
O
CO p* empty antibonding
acceptor orbital
s/p Bridging CO’s
O
C
M
O
O
C
C
M
M
M
M
M
M
CO acting as p-donor or p-acceptor?
1.30Å
2.22Å
O
C
C
C
Nb
Nb
Cp
O
2.25Å
O
C
O
Cp
Nb
C
O
1.97Å
Cp
C
O
C
M
O
Herrman & coworkers
JACS, 1981, 103, 1692
Problem: Which of the following metal carbonyl IR spectra represents the
compound with the least amount of electron density on the metal center?
Briefly discuss the reasoning for your choice. Which compound will lose CO
the easiest?
Problem: Which of the following metal carbonyl compounds will have the
highest nCO stretching frequency in the IR? Why? Will this be the most
electron-rich or deficient compound?
a)
b)
CO
F
Ir
F
CO
Br
CO
CO
Ir
Br
F
CO
Br
c)
CO
Me2N
Me2N
Ir
CO
CO
NMe2
CO